Sat-navs are almost ALWAYS more accurate than the car speedo. By law, a mass-produced car speedo is allowed to OVER-read by 10%+2.5MPH more than the true speed, but it must NOT under-read under ANY circumstances. As the vehicles are mass-produced, they need to build some tolerance into the speedo calibration to allow for natural small variations between vehicles, so they always over-read by a bit. Different brands of tyre (even of the same size) will have a slightly different rolling radius, so they need to cater for the possibility of someone fitting 4 brand new ones of a bigger diameter and running them at maximum pressure. Finally, in vehicles where more than one size of tyre could be fitted, manufacturers will sometimes save money by keeping the same speed calibration in both - so one might over-read by a bit more than the other but both would still be within the law.
From personal experience, 5MPH at 70 is a fair bit for an excess reading. Most modern cars that I've had anything to do with are more like 2-3MPH over the odds at that speed.
Only thing I would say about sat-navs is that you need to be going in a straight line at roughly constant speed for several seconds to get the best out of them, because their own reaidngs will vary slightly every second or so.
Oh, and if it helps, I too know someone who got done for 79 on a clear, dry, and near-empty M6 in Cumbria too! They really are quite zealous (with very little to show for it in terms of accident reduction!) but this might cheer you up!
http://www.newsandstar.co.uk/news/redun ... rPath=news
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